Closed type inwardly-opening fuel injection nozzle assemblies typically include a hollow spray tip or housing and a flow check positioned in the tip. The tip has one or more fuel spray orifices and an internal tip seat upon which the movable check selectively seats.
One category of such nozzle assemblies, known as sac-type nozzle assemblies, generally describes a tip configuration wherein the orifices are located through a sac projecting from the apex of the tip. Thus, in a sac-type tip, the orifices are remotely spaced from the tip seat such that the check does not cover, or even partially cover, the upstream entrances of the orifices when the check is seated on the tip seat. Examples of known sac-type nozzle assemblies are shown in U.S. Pat. No. 3,391,871 issued to Fleischer et al. on Jul. 9, 1968 and U.S. Pat. No. 4,527,738 issued to Martin on Jul. 9, 1985. Sac-type nozzle assemblies having a relatively small sac volume are known as mini-sac nozzle assemblies. An example of a mini-sac nozzle assembly is shown in U.S. Pat. No. 5,037,031 issued to Campbell et al. on Aug. 6, 1991.
Typically, the sac of a sac-type tip has a wall thickness in the range of about 0.60 to 0.80 mm or millimeters (about 0.024 to 0.031 inches) in the region where the orifices pass through. The ratio of the axial length of an orifice to its cross-sectional diameter helps determine its spray characteristics. Generally, a relatively shorter length orifice produces a bushier fuel spray having a relatively lower penetration capability through air in a combustion chamber compared to a relatively longer length orifice of the same cross-sectional area. Sac-type tips generally produce well-atomized fuel sprays or plumes which effectively disperse fuel over a wide region to facilitate good mixing with air present in the engine combustion chamber.
However, sac-type tips are becoming undesirable for currently-produced engines because such tips help produce particulates that may prevent the engines from meeting current and/or future stringent emissions standards. The main culprit is existence of the relatively large volume sac which contains fuel after the check has seated on the tip seat to end injection. Such fuel remaining in the sac, after the check is seated, may continue flowing at a reduced pressure towards the uncovered entrances of each orifice due to fluid momentum and/or thermal expansion caused by heat transfer from the engine combustion chamber. Such fuel may dribble out of the orifices and into the engine combustion chamber as an non-atomized fuel stream at an undesirable time in the engine cycle resulting in particulate emissions.
Another category of such nozzle assemblies, known as valve-closed-orifice (VCO) nozzle assemblies, generally describes a tip configuration in which the upstream entrance of each orifice either i) intersects the tip seat or ii) is located downstream of the tip seat but is adjacent to or in close proximity to the tip seat. Another definition of a VCO nozzle assembly is that the combined exposed cross-sectional flow areas at the upstream entrance to each orifice in the tip is either i) zero when the check is seated on the tip seat or ii) is at least less than the combined exposed cross-sectional flow areas at the upstream entrance to each orifice when the check is unseated from the tip seat. Typically, the upstream entrance to each orifice is entirely or at least partially covered by the check when the check is seated on the tip seat. Examples of known VCO nozzle assemblies are shown in U.S. Pat. No. 4,083,498 issued to Cavanagh et al. on Apr. 11, 1978, U.S. Pat. No. 4,540,126 issued to Yoneda et al. on Sep. 10, 1985, and U.S. Pat. No. 4,715,541 issued to Freudenschuss et al. on Dec. 29, 1987.
VCO nozzle assemblies have certain advantages over sac-type nozzle assemblies which make the former desirable for helping currently produced engines meet stringent emission standards. First, the location of the orifices in a VCO tip eliminates the need for a sac to accommodate such orifices and the fuel flowpath thereto. Elimination of the sac minimizes the amount of fuel remaining in the tip downstream or below the check after the check has seated on the tip seat. Moreover, after injection has ended and the check becomes seated on the tip seat, any fuel remaining in the tip downstream of the check is prevented or at least inhibited from simply dribbling into the engine combustion chamber since the upstream entrance of each orifice is either covered or at least partially covered by the seated check.
A problem with VCO nozzle assemblies has been that the relatively closer proximity of the orifices to the tip seat has been traditionally thought to produce a significantly high stress concentration factor in that region. The conventional approach to coping with such perceived high stress has been to increase the wall thickness of the VCO tip in that region.
The minimum allowable VCO tip wall thickness has been traditionally determined with the aid of a stress concentration curve plotting stress concentration factor, k.sub.c, as a function of orifice angle, theta. As shown in FIG. 4, orifice angle means the included angle between the tip seat and the centerline axis of the respective orifice. A previously known stress concentration curve is labeled as curve k.sub.c1 in FIG. 3. This curve was generated by a simple three-dimensional analysis.
For example, some engine cylinder head configurations having a fuel injector, one exhaust valve and one air intake valve require that the fuel injector to be installed at an angle, relative to the piston centerline axis, with the orifices positioned in the tip in an oblique pattern relative to the piston centerline. In other words, the orifice angles must be made less than 90.degree.. As the orifice angle theta decreases, the previously known k.sub.c1 curve of FIG. 3 predicts a higher stress concentration factor in the region of the tip seat/orifice intersection. Traditionally, the wall thickness of the VCO tip in this region has been increased to a thickness far in excess of the above-mentioned typical wall thicknesses for the sac of a sac-type tip. For example, as stated in U.S. Pat. No. 5,016,820 issued to Gaskell on May 21, 1991 and U.S. Pat. No. 5,092,039 issued to Gaskell on Mar. 3, 1992, there is a strict limit to how far the wall thickness of a nozzle can be reduced in the case of VCO nozzles, on grounds of strength; with the high injection pressures involved, there is a danger of the tip of the nozzle being blown off if it is of inadequate strength. Gaskell says in practice the wall thickness must be 1 mm (0.0394 inches) or at the very least 0.8 mm (0.315 inches). The preceding statement and FIG. 1 of Gaskell suggests that the above stated 0.8 mm minimum wall thickness is for an orifice angle, theta, of 90.degree.. For orifice angles theta less than 90.degree. one of ordinary skill in the art traditionally concludes that the corresponding minimum wall thickness should be greater than the 0.8 mm wall thickness indicated for theta equal to 90.degree..
One disadvantage of such relatively thick walled VCO tips is the increased cost of forming orifices through such tips. Another disadvantage is that the relatively thick wall of a VCO tip may produce poor fuel spray characteristics which undesirably result in higher emissions. The reason for higher emissions is that a relatively thick walled VCO tip consequently results in a relatively longer orifice length such that the orifice acts somewhat like a long-barreled rifle when injecting fuel. During fuel injection, the fuel exiting the long orifice remains as a relatively concentrated fuel stream instead of sufficiently atomizing and mixing with the air present in an engine combustion chamber. In relatively small engine combustion chambers, such concentrated fuel streams may undesirably impinge on the piston or cylinder bore resulting in emissions.
The present invention is directed to overcoming one or more of the problems as set forth above.